Solid-state lighting devices made of light-emitting diodes (LEDs) are increasingly useful for applications requiring robustness and long life. For example, solid-state LEDs are found today in automotive applications. These devices are typically formed by combining multiple, small LED devices providing a point light source into a single module together with glass lenses or reflectors suitably designed to direct the light as is desired for a particular application; see for example, WO99/57945, published Nov. 11, 1999. These multiple devices are expensive and complex to manufacture and integrate into single illumination devices. Moreover, point sources of light such as LEDs or incandescent lamps tend to require additional light diffusers, e.g. lampshades, to avoid glare.
Organic light-emitting diodes (OLEDs) are manufactured by depositing organic semiconductor materials between electrodes on a substrate. This process enables the creation of area-emitting light sources having an extended light-emitting surface area on a single substrate, thereby reducing glare and improving the efficiency of illumination. The prior art describes the use of one or more OLEDs in lighting, for example U.S. Pat. No. 6,565,231, entitled “OLED Area Illumination Lighting Apparatus” filed by Cok on May 28, 2002. In particular, the use of specially constructed layers within an OLED device to form optical cavities that can enhance the amount of light output from an OLED device is known. For example, U.S. Application Publication No. 2004/0155576 filed Feb. 4, 2004, entitled “Microcavity OLED Devices” describes such an OLED device. Other means, such as diffraction gratings can be employed to similar effect. Because these layers in these devices provide an optical filtering effect, the layers are carefully selected to minimize frequency dependence on the angle of emission for the emitted light. Alternatively, scattering or diffusing elements are employed to maintain a consistent color of light emitted over the surface of the OLED device and at any viewed angle.
A skilled artisan appreciates that these techniques involve filtering, reflecting or otherwise processing light generated by the OLED device and that with each such processing step, a portion of the light generated by the OLED device can be subject to optical losses, thus the overall efficiency of such an illumination system as measured in terms of lumens per watt of supplied energy may decrease. Furthermore, the efficiency of an area-illumination system can be a critical feature in the selection of one form of area illumination as compared against other potential forms of area illumination, particularly, where vast areas such as roadways, athletic stadiums, or other areas are to be illuminated. Moreover, each processing step may incur additional manufacturing costs.
Another factor in the selection of an area-illumination system is the aesthetic appeal or lack thereof of the selected area-illumination system itself. Colored lights are sometimes employed as decoration or specialty lighting, for example, as holiday lighting. Colored illumination is typically provided using filters over white-light lamps. See for example, US 2004/0090787 entitled “Methods and Systems for Illuminating Environments” published May 13, 2004; US 2004/0052076 entitled “Controlled Lighting Methods and Apparatus” published Mar. 18, 2004; and US 2004/0105264 “Multiple Light-Source Illuminating System”, published Jun. 3, 2004. However, it can be appreciated that here too, the filters absorb light, reduce the efficiency of the illumination system, and cause the area illuminated thereby to take on the colors of the filtered light.
A solid-state area illumination system has been described by Cok in US 2007/0126004A1 “Lamp with Multi-Colored OLED Elements”, published Jun. 7, 2007, comprising a plurality of OLED devices, each device formed on a separate substrate and each device emitting light at a plurality of angles relative to the substrate, the emitted light having different ranges of frequencies at different ranges of the plurality of angles; and a support positioning each of the plurality of OLED devices at a plurality of orientations relative to an area of illumination, the positioning being defined so that any point on any surface within the area of illumination receives a broadband combination of light from more than one of the OLED devices. In this system, light filtering and processing steps are eliminated, so that efficient use of light is maintained. Further, the angular variation of emission color associated with thin-film electroluminescent devices is used to facilitate color mixing, so that each point on a surface is illuminated diffusely by a combination of colors, each color having emanated from a different OLED device at a different illumination angle, resulting in white light at the surface point. The OLED solid-state area-illumination system of US 2007/0126004A1 does not clearly distinguish colors at different angles of emission from the substrate, because of the broadband emission therein. This may lead to reduced aesthetic value in that a perceptively reduced range and saturation of colors is produced when viewing the OLED devices. Therefore, the colors are reduced in variation and are not easily distinguished from one another. Moreover, the OLED solid-state area illumination system of US 2007/0126004A1 may have shorter lifetimes than conventional illumination, lower efficiencies, and require expensive encapsulation for device operation. Therefore, an alternative technology having improved performance is still desired.